U.S. patent application number 11/510883 was filed with the patent office on 2007-03-08 for metal duplex method.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Raymund W.M. Kwok, Danny Lau.
Application Number | 20070052105 11/510883 |
Document ID | / |
Family ID | 37564172 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070052105 |
Kind Code |
A1 |
Lau; Danny ; et al. |
March 8, 2007 |
Metal duplex method
Abstract
Methods and articles are disclosed. The methods are directed to
depositing nickel duplex layers on substrates to inhibit corrosion
and improve solderability of the substrates. The substrates have a
gold or gold alloy finish.
Inventors: |
Lau; Danny; (Hong Kong,
HK) ; Kwok; Raymund W.M.; (Laguna City, HK) |
Correspondence
Address: |
John J. Piskorski;Rohm and Haas Electronic Materials LLC
455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
|
Family ID: |
37564172 |
Appl. No.: |
11/510883 |
Filed: |
August 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60714575 |
Sep 7, 2005 |
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Current U.S.
Class: |
257/766 ;
257/E23.01; 257/E23.054; 257/E23.06 |
Current CPC
Class: |
H01L 2924/0104 20130101;
H01L 2924/01082 20130101; C23C 28/023 20130101; H01L 2924/014
20130101; H01L 2224/13099 20130101; H01L 2924/0105 20130101; H01L
2224/2919 20130101; H01L 2924/01019 20130101; H01L 2924/01029
20130101; H01L 2924/19043 20130101; H01L 2924/14 20130101; H01L
2924/12042 20130101; C23C 18/32 20130101; H01L 2924/01028 20130101;
H01L 2224/45124 20130101; H01R 13/03 20130101; C25D 5/14 20130101;
C25D 5/505 20130101; H01L 23/49582 20130101; H01L 2924/01005
20130101; Y10S 428/929 20130101; H01L 2924/01078 20130101; H01L
24/10 20130101; H01L 2924/01052 20130101; H01L 2924/01011 20130101;
H01L 2924/01051 20130101; H05K 3/244 20130101; H01L 2924/00014
20130101; H01L 2924/19041 20130101; H01L 2924/01033 20130101; H01L
2924/01012 20130101; H01L 2924/01049 20130101; H01L 2924/01046
20130101; H01L 2924/01047 20130101; H01L 2924/01084 20130101; C23C
28/021 20130101; H01L 2224/45144 20130101; Y10T 428/24917 20150115;
H01L 2924/01006 20130101; H01L 23/498 20130101; H01L 2924/01015
20130101; C23C 18/1653 20130101; H01L 2224/13 20130101; H01L
2924/01016 20130101; Y10T 428/1291 20150115; H01L 24/13 20130101;
H01L 2924/01004 20130101; H01L 2924/01027 20130101; H01L 2924/0103
20130101; H01L 2924/01014 20130101; H01L 2924/01327 20130101; C23C
18/1651 20130101; C23C 28/028 20130101; H01L 2924/01322 20130101;
H01L 2924/01013 20130101; H01L 2924/01079 20130101; Y10T 428/12722
20150115; H01L 24/45 20130101; Y10S 428/939 20130101; H01L
2224/29339 20130101; H01L 2924/01025 20130101; H01L 2224/45124
20130101; H01L 2924/00014 20130101; H01L 2224/45144 20130101; H01L
2924/00014 20130101; H01L 2224/2919 20130101; H01L 2924/0665
20130101; H01L 2924/00014 20130101; H01L 2224/48 20130101; H01L
2224/13 20130101; H01L 2924/00 20130101; H01L 2924/12042 20130101;
H01L 2924/00 20130101 |
Class at
Publication: |
257/766 ;
257/E23.01 |
International
Class: |
H01L 23/48 20060101
H01L023/48 |
Claims
1. A method comprising depositing a nickel layer on a substrate,
depositing a nickel phosphorous layer on the nickel layer and
depositing a gold or gold alloy layer on the nickel phosphorous,
the nickel phosphorous layer comprises 0.1 wt % to 10 wt % of
phosphorous.
2. The method of claim 1, wherein the nickel phosphorous layer
comprises 1 wt % to 9 wt % of phosphorous.
3. A method comprising depositing 1 micron to 10 microns of a
nickel layer on a substrate, depositing 0.1 microns to 5 microns of
a nickel phosphorous layer on the nickel layer, and depositing a
gold or gold alloy layer on the nickel phosphorous layer, the
nickel phosphorous layer comprises 1 wt % to 10 wt % of
phosphorous.
4. The method of claim 3, further comprising the step of thermal
aging the gold or gold alloy layer.
5. An article comprising a nickel layer on a substrate and a nickel
phosphorous layer on the nickel layer a gold or gold alloy on the
nickel phosphorous layer, the nickel phosphorous layer comprises
0.1 wt % to 10 wt % of phosphorous.
6. The article of claim 5, wherein the nickel phosphorous layer
comprises 1 wt % to 9 wt % of phosphorous.
7. The article of claim 5, wherein the nickel layer is 1 micron to
10 microns thick and the nickel phosphorous layer is 0.1 microns to
5 microns thick.
8. The article of claim 5, wherein the article is a printed wiring
board, a lead frame, a connector, wafer bump or passive component.
Description
[0001] The present invention is directed to a low phosphorous
containing metal duplex with a gold or gold alloy layer and method
of making the low phosphorous metal duplex with the gold or gold
alloy layer. More specifically, the present invention is directed
to a low phosphorous containing metal duplex with a gold or gold
alloy layer and method of making the low phosphorous metal duplex
with the gold or gold alloy layer for inhibiting corrosion and
improving solderability.
[0002] Prevention of corrosion of gold and gold alloys is a
challenging problem in numerous industries. The corrosion of gold
and gold alloys has been especially problematic in the electronic
materials industry where corrosion may lead to faulty electrical
contact between components in electronic devices. For example, gold
and gold alloy coatings have been used in electronics and other
applications for many years. In electronics they have been used as
a solderable and corrosion resistive surface for contacts and
connectors. They are also used in lead finishes for integrated
circuit (IC) fabrication. However, gold finishes may not always
prevent corrosion.
[0003] IC devices, having IC units, lead frames and passive
components, such as capacitors and transistors, find wide use in
products including consumer electronics, household appliances,
computers, automobiles, telecommunications, robotics and military
equipment. The IC unit encompasses IC chips and hybrid circuit
nodules which include one or more of the IC chips and other
electronic components on a plastic or ceramic support base.
[0004] Lead frames or connectors are a means to electrically
interconnect an IC unit to external circuitry. The lead frame is
formed from electrically conductive material such as copper or
copper alloys, or by stamping or etching a metal blank into a
plurality of leads defining a central area in which the IC unit is
mounted. There are several attachment techniques by which the lead
frames connect the IC units in an assembly. These include
wirebonding, soldering, die-attach and encapsulation. Typically
soldering is the means of joining the IC to the assembly. In all
instances attachment requires a particular quality of the lead
frame surface. Typically this means that the surface is corrosion
free and ready for interaction with other components such as gold
or aluminum wire, silver filled epoxy or solder.
[0005] One problem is long term solderability, defined as the
ability of the surface finish to melt and make a good solder joint
to other components without defects that impair the electrical or
mechanical connection. Generally, soldering is an attachment
procedure that usually involves three materials: (1) the substrate,
(2) the component or other device which is desired to be attached
to the substrate, and (3) the soldering material itself.
[0006] There are many factors that determine good solderability,
the three most important of which are extent of corrosion, amount
of co-deposited carbon, and extent of intermetallic compound
formation. Corrosion is a natural occurring process because it is
thermodynamically favorable. The rate of corrosion depends on
temperature and time. The higher the temperature and the longer the
exposure time, the thicker the corrosion.
[0007] Co-deposited carbon is determined by the plating chemistry
one chooses to use. Bright finishes contain higher carbon contents
than matte finishes. Matte finishes are normally rougher than
bright finishes, and provide an increased surface area which
results in the formation of more corrosion than typically are
formed with bright finishes.
[0008] Intermetallic compound formation is a chemical reaction
between metal coating and the underlying metal substrate. The rate
of formation depends on temperature and time as well. Higher
temperatures and longer times result in a thicker layer of
intermetallic compounds.
[0009] Certain physical properties of gold, such as its relative
porosity, translate into problems when gold is deposited onto a
substrate. For instance, gold's porosity can create interstices on
the plated surface. These small spaces can contribute to corrosion
or actually accelerate corrosion through the galvanic coupling of
the gold layer with the underlying base metal layer. This is
believed to be due to the base metal substrate and any accompanying
underlying metal layers which may be exposed to corrosive elements
via the pores in the gold outer surface.
[0010] In addition, many applications include thermal exposure of
coated lead frames. Diffusion of metal between layers under thermal
aging conditions may cause a loss of surface quality if an
underlying metal diffuses into a noble metal surface layer such as
nickel into gold.
[0011] At least three different approaches to overcoming the
corrosion problems have been attempted: 1) reducing the porosity of
the coating, 2) inhibiting the galvanic effects caused by the
electropotential differences of different metals, and 3) sealing
the pores in the electroplated layer. Reducing the porosity has
been studied extensively. Pulse plating of the gold and utilization
of various wetting/grain refining agents in the gold plating bath
affect the gold structure and are two factors contributing to a
reduction in gold porosity. Often regular carbon bath treatments
and good filtration practices in the series of electroplating baths
or tanks combined with a preventive maintenance program help to
maintain gold metal deposition levels and correspondingly low
levels of surface porosity. A certain degree of porosity, however,
continues to remain.
[0012] Pore closure, sealing and other corrosion inhibition methods
have been tried but with limited success. Potential mechanisms
using organic precipitates having corrosion inhibitive effects are
known in the art. Many of these compounds were typically soluble in
organic solvents and were deemed not to provide long term corrosion
protection. Other methods of pore sealing or pore blocking are
based on the formation of insoluble compounds inside pores. Such
insoluble complexes and precipitates as are apparent to those of
skill in the field also are potential candidates for pore
blocking.
[0013] In addition to the problem of pore formation, exposing gold
to elevated temperatures, such as in thermal aging, undesirably
increases the gold's contact resistance. This increase in contact
resistance compromises the performance of the gold as a conductor
of current. In theory, workers believe that this problem arises
from the diffusion of organic materials co-deposited with the gold
to the contact surfaces. Various techniques for obviating this
problem have been attempted heretofore, typically involving
electrolytic polishing. However, none have proven completely
satisfactory for this purpose and investigative efforts
continue.
[0014] U.S. Pat. No. 6,287,896 to Yeh et al. discloses a thick
nickel-cobalt alloy coating a copper or copper alloy base plate.
The patent states that the thick nickel-cobalt layer is essential
for reducing the effects of porosity and prevents the diffusion of
copper from the base plate to the surface which is used for
soldering. A nickel or nickel alloy coats the thick nickel-cobalt
layer to prevent it from cracking, an inherent problem of the
nickel-cobalt layer. A gold or gold alloy layer coats the nickel or
nickel alloy layer. The gold or gold alloy layer allegedly improves
solderability.
[0015] Although there are methods which attempt to address the
corrosion and solderability problems on metal deposits, there is
still a need for improved methods for inhibiting corrosion and
improving solderability.
[0016] Methods include depositing a nickel layer on a substrate,
depositing a layer of nickel phosphorous on the nickel layer and
depositing a layer of gold or gold alloy on the nickel phosphorous
layer, the nickel phosphorous layer includes 0.1 wt % to 10 wt %
phosphorous. The combination of the nickel and nickel phosphorous
layers inhibits or prevents the effects of thermal ageing, thus
decreasing pore formation in the gold or gold alloy layer. The
reduction of pores in the gold or gold alloy layer reduces or
prevents corrosion of the underlying substrate. Solderability of
the gold or gold alloy finish is thus improved.
[0017] In another aspect, methods include depositing 1 micron to 10
microns of a nickel layer on a substrate, depositing 0.1 micron to
5 microns of nickel phosphorous layer on the nickel layer, and
depositing a gold or gold alloy layer on the nickel phosphorous
layer, the nickel phosphorous layer includes 0.1 wt % to 10 wt %
phosphorous.
[0018] In a further aspect, articles include a nickel layer on a
substrate, a nickel phosphorous layer on the nickel layer and a
gold or gold alloy layer on the nickel phosphorous layer, the
nickel phosphorous layer includes 0.1 wt % to 10 wt %
phosphorous.
[0019] FIG. 1 is a photograph 50.times. of a gold layer with pores
on a nickel underlayer; and
[0020] FIG. 2 is a photograph 50.times. of a gold layer with pores
on a nickel phosphorous underlayer.
[0021] As used throughout this embodiment, the following
abbreviations have the following meanings, unless the context
clearly indicates otherwise: .degree. C.=degrees Centigrade;
g=gram; mg=milligram; L=liter; ml=milliliter;
.ANG.=angstroms=1.times.10.sup.-4 microns; ASD=amperes/dm.sup.2;
and wt %=weight percent. The terms "depositing" and "plating" are
used interchangeably throughout this specification. All numerical
ranges are inclusive and combinable in any order, except where it
is logical that such numerical ranges are constrained to add up to
100%.
[0022] Methods include depositing a nickel layer on a substrate,
depositing a layer of nickel phosphorous on the nickel layer and
depositing a gold or gold alloy layer on the nickel phosphorous
layer, the nickel phosphorous layer includes 0.1 wt % to 10 wt % of
phosphorous. The combination of the nickel and nickel phosphorous
layers reduces the effects of thermal aging, thus decreasing pore
formation in gold and gold alloy layers. The reduction of pore
formation in turn reduces or eliminates corrosion of the underlying
metal substrate and also improves solderability of the gold or gold
alloy layer. The nickel and nickel phosphorous layers also reduce
or inhibit the migration of nickel into the gold or gold alloy
layer.
[0023] Optionally, prior to metallization the substrate may be
cleaned. Any suitable cleaning process may be used which is
acceptable in the metallization arts. Typically, the substrate is
ultrasonically cleaned in a cleaning solution. Such cleaning
solutions may include silicate compounds, alkali metal carbonates
and other compounds such as alkali metal hydroxides, glycol ethers
and one or more chelating agents. Cleaning may be done at
temperatures of from 30.degree. C. to 80.degree. C.
[0024] Optionally, following the cleaning step, the substrate may
be activated with a suitable acid such as a mineral acid. Dilute
concentrations of mineral acids are used. An example of such an
acid is sulfuric acid. However, other mineral acids may be used
such a hydrochloric acid and nitric acid. The acids are used at
conventional concentrations well known in the art. Activation
typically is done at temperatures from room temperature to
30.degree. C.
[0025] The substrate is then plated with a deposit of nickel metal.
Bath temperatures range from 30.degree. C. to 70.degree. C. or such
as from 40.degree. C. to 60.degree. C.
[0026] Any suitable nickel plating bath may be used to deposit the
nickel layer on the substrate. Such nickel plating baths include
one or more sources of nickel ions. Sources of nickel ions include,
but are not limited to, nickel halides such as nickel chloride,
nickel sulfate and nickel sulfamate. Such sources of nickel ions
are included in the nickel baths in amounts of 50 gm/L to 500 gm/L,
or such as from 100 gm/L to 250 gm/L.
[0027] In addition to the one or more sources of nickel ions, the
nickel plating baths may include one or more additives. Such
additives include, but are not limited to, brighteners, grain
refiners, levelers, surface active agents, antipitting agents,
chelating agents, buffers, biocides and other additives known to
those of skill in the art to tailor the bath to a desired bright or
matt finish and throwing power.
[0028] Brighteners include, but are not limited to, aromatic
sulfonates, sulfonamides, sulfonimides, aliphatic and
aromatic-aliphatic olefinically or acetylenically unsaturated
sulfonates, sulfonamides and sulfonimides. Examples of such
brighteners are sodium o-sulfobenzimide, disodium 1,5-naphthalene
disulfonate, trisodium 1,3,6-naphthalene trisulfonate, sodium
benzene monosulfonate, dibenzene sulfonamide, sodium allyl
sulfonate, sodium 3-chloro-2-butene-1-sulfonate, sodium
.beta.-styrene sulfonate, sodium propargyl sulfonate, monoallyl
sulfamide, diallyl sulfamide and allyl sulfonamide. Such
brighteners may be used in conventional amounts such as 0.5 g/L to
10 g/L, or such as 2 g/L to 6 g/L.
[0029] Other brighteners include, but are not limited to, reaction
products of epoxides with alphahydroxy acetylenic alcohols such as
diethoxylated 2-butyne-1,4-diol or dipropoxylated
2-butyne-1,4-diol, N-heterocyclics, other acetylenic compounds,
active sulfur compounds and dye-stuffs. Examples of such
brighteners are 1,4-di-(.beta.-hydroxyethoxy)-2-butyne,
1,4-di-(.beta.-hydroxy-.gamma.-chloropropoxy)-2-butyne,
1,4-di-(.beta.-,.gamma.-epoxypropoxy)-2-butyne,
1,4-di-(.beta.-hydroxy-.gamma.-butenoxy)-2-butyne,
1,4-di-(2'-hydroxy-4'-oxa-6'-heptenoxy)-2-butyne,
N-(2,3-dichloro-2-propenyl)-pyridinium chloride, 2,4,6-trimethyl
N-propargyl pyridinium bromide, N-allylquinaldinium bromide,
2-butyne-1,4-diol, propargyl alcohol, 2-methyl-3-butyn-2-ol,
quinaldyl-N-propanesulfonic acid betaine, quinaldine dimethyl
sulfate, N-allylpyridinium bromide, isoquinaldyl-N-propanesulfonic
acid betaine, isoquinaldine dimethyl sulfate, N-allylisoquinaldine
bromide, disulfonated 1,4-di-(.beta.-hydroxyethoxy)-2-butyne,
1-(.beta.-hydroxyethoxy)-2-propyne,
1-(.beta.-hydroxypropoxy)-2-propyne, sulfonated
1-(.beta.-hydroxyethoxy)-2-propyne, phenosafranin and fuchsin. Such
brighteners are included in conventional amounts such as, for
example, 5 mg/L to 1000 mg/L, or such as from 20 mg/L to 500
mg/L.
[0030] Any suitable surface active agent may be used. Such surface
active agents include, but are not limited to, ionic surfactants
such as cationic and anionic surfactants, non-ionic surfactants and
amphoteric surfactants. Surfactants may be used in conventional
amounts such as, for example, 0.05 gm/L to 30 gm/L, or such as from
1 gm/L to 20 gm/L or such as from 5 gm/L to 10 gm/L.
[0031] An example of a suitable surfactant is naphthalene
sulfonated with 1 to 8 sulfonic acid groups (--SO.sub.3H). Examples
of such surfactants are naphthalene-1,3,6-trissulfonic acid and
naphthalene-1,3,7-trissulfonic acid. The alkali metal salts such as
the sodium and potassium salts also may be used.
[0032] Examples of other suitable surfactants are salts of alkyl
hydrogen sulfates such as sodium lauryl sulfate, sodium lauryl
ether-sulfate and sodium di-alkylsulfosuccinates. Examples of other
surfactants which may be used are quaternary ammonium salts
including perfluorinated quaternary amines such as perfluoro
dodecyl trimethyl ammonium fluoride.
[0033] Suitable chelating agents include, but are not limited to,
amino carboxylic acids, polycarboxylic acids and polyphosphonic
acids. Such chelating agents may be used in conventional amounts
such as 0.01 moles/L to 3 moles/L or such as from 0.1 moles/L to
0.5 moles/L.
[0034] The pH of the nickel baths may range from 1 to 10, or such
as from 3 to 8. The pH of the nickel baths may be maintained by a
variety of means. Any desired acid or base may be used, and any
inorganic acid, organic acid, inorganic base, or organic base may
be used. Besides acids such as sulfuric acid, hydrochloric acid, or
sulfamic acid, acids used as chelating agents such as acetic acid,
amino acetic acid or ascorbic acid also may be used. Besides
inorganic bases such as sodium hydroxide or potassium hydroxide and
organic bases such as various types of amines, bases such as nickel
carbonate also may be used. In addition, a pH buffering ingredient
such as boric acid may be used if the pH tends to fluctuate due to
operating conditions. The buffers may be added in amounts as needed
to maintain a desired pH.
[0035] Other additives may be added to the nickel metal plating
baths which are conventional and well known those of skill in the
art. They are used in conventional amounts to tailor the nickel
layer to a desired matt, semi-bright or bright finish.
[0036] Nickel is deposited on the substrate until a nickel layer of
from 1 micron to 10 microns, or such as from 2 microns to 5 microns
is formed on the substrate. The nickel layer is then plated with a
layer of nickel phosphorous using a nickel phosphorous bath.
[0037] The nickel phosphorous bath includes one or more sources of
nickel ions as described above, and may include one or more of the
additives as described above. In addition, the nickel phosphorous
bath includes one or more sources of phosphorous. Any suitable
phosphorous acid or phosphoric acid or salt thereof and mixtures
may be used. Phosphorous acids and phosphoric acids and their salts
are included in the baths in amounts of 5 gm/L to 100 gm/L, or such
as from 10 gm/L to 80 gm/L, or such as from 20 gm/L to 50 gm/L.
Phosphorous acid has the general formula: H.sub.3PO.sub.3 and also
is known as orthophosphorous acid. Phosphoric acids include, but
are not limited to, inorganic phosphoric acids such as phosphoric
acid (H.sub.3PO.sub.4), also known as orthophosphoric acid.
Polyphosphoric acids also may be used. Inorganic phosphoric acids
may be represented by the formula: H.sub.n+2P.sub.nO.sub.3n+1,
where n is an integer of 1 or greater. When n is an integer of 2 or
greater, the formula represents a polyphosphoric acid. When the
inorganic phosphoric acid is a polyphosphoric acid, typically, n is
an integer such that the polyphosphoric acid has an average
molecular weight of 110 to 1,500 atomic weight units. Typically,
phosphorous acid is used.
[0038] Salts of phosphoric acids such as alkali metal phosphates
and ammonium phosphate may be used. Alkali metal phosphates include
dibasic sodium phosphate, tribasic sodium phosphate, dibasic
potassium phosphate and tribasic potassium phosphate. Salts of the
polyphosphoric acids also may be used. Mixtures of the inorganic
phosphoric acids and their salts also may be used. Such acids are
commercially available or may be made according to methods
described in the literature.
[0039] The nickel phosphorous layer is deposited on the nickel
layer at the same bath temperatures as the nickel layer is
deposited on the substrate. Deposition continues until a nickel
phosphorous layer of 0.1 microns to 5 microns or such as from 0.2
microns to 1 micron is deposited on the nickel layer to form a
duplex.
[0040] Typically, the weight ratio of nickel to nickel phosphorous
in the duplex is from 2:1 to 8:1. The nickel and nickel phosphorous
layers may be deposited by any suitable electrolytic deposition
method known in the art. Conventional plating apparatus may be used
to deposit the nickel and nickel phosphorous layers. Current
densities may range from 1 ASD to 20 ASD, or such as from 5 ASD to
15 ASD.
[0041] Typically, the nickel and nickel phosphorous duplex layer is
2 microns to 3 microns thick. After the nickel and nickel
phosphorous layers are deposited on the substrate, a surface finish
of gold or gold alloy is deposited on the nickel phosphorous
layer.
[0042] The article formed by the methods described above includes a
substrate plated with a nickel layer and a nickel phosphorous layer
over the nickel layer to form the duplex. Phosphorous content of
the nickel phosphorous layer may range from 0.1 wt % to 10 wt %, or
such as from 0.5 wt % to less than 10 wt %, or such as from 1 wt %
to 9 wt %, or such as from 2 wt % to 8 wt %, or such as from 3 wt %
to 7 wt %. A gold or gold alloy layer is deposited over the nickel
phosphorous layer to form an article. The duplex layers composed of
the nickel and nickel phosphorous layers inhibit pore formation in
the gold or gold alloy layer and inhibit nickel migration into the
gold or gold alloy layer. Connectors soldered to the gold or gold
alloy coated articles are more secure and less likely to separate
than gold or gold alloy coated articles without the duplex
layer.
[0043] Gold or gold alloy may be deposited on the nickel
phosphorous layer using any suitable source of gold ions as well as
alloying metal ions. Sources of gold ions include, but are not
limited to, sodium dicyanoaurate (I), ammonium dicyanoaurate (I)
and other dicyanoauric acid (I) salts; potassium tetracyanoaurate
(III), sodium tetracyanoaurate (III), ammonium tetracyanoaurate
(III) and other tetracyanoauric acid (III) salts; gold (I) cyanide,
gold (III) cyanide; dichloroauric acid (I) salts; tetrachloroauric
acid (III), sodium tetrachloroaurate (III) and other
tetrachloroauric acid (III) compounds; ammonium gold sulfite,
potassium gold sulfite, sodium gold sulfite and other sulfurous
acid gold salts; gold oxide, gold hydroxide and other alkali metal
salts thereof; and nitrosulphito gold complexes. The sources of
gold typically are included in conventional amounts such as 0.1
gm/L to 10 g/L or such as from 1 gm/L to 5 gm/L.
[0044] Alloying metals include, but are not limited to, copper,
nickel, zinc, cobalt, silver, the platinum group metals, cadmium,
lead, mercury, arsenic, tin, selenium, tellurium, manganese,
magnesium, indium, antimony, iron, bismuth and thallium. Typically
the alloying metal is cobalt. Sources of such alloying metals are
well known in the art. The sources of alloying metals are included
in the bath in conventional amounts and vary widely depending on
the type of alloying metal used.
[0045] In addition to the sources of gold and alloying metals the
gold and gold alloy baths may include one or more additives. Such
additives include, but are not limited to, surfactants,
brighteners, levelers, complexing agents, chelating agents, buffers
and biocides. Such additives are included in conventional amounts
and are well known to those of skill in the art.
[0046] The gold or gold alloy may be deposited on the nickel
phosphorous layer by any suitable method known in the art. Such
methods include, but are not limited to, electrolytic deposition
and immersion deposition. Typically, the gold or gold alloy is
deposited electrolytically. Current densities for depositing gold
and gold alloys electrolytically range from 1 ASD to 30 ASD, or
such as form 5 ASD to 20 ASD.
[0047] After the gold or gold alloy is deposited on the nickel
phosphorous layer, typically it undergoes thermal aging. Thermal
aging may be done by any suitable method known in the art.
Presently, there are no standard thermal aging requirements and
various methods are used. Such methods include, but are not limited
to, steam aging and dry baking. The nickel and nickel phosphorous
duplex inhibits surface diffusion of nickel into the gold surface
finish, thus solderability is improved.
[0048] In addition to preventing the diffusion of less noble metals
into gold and gold alloys, the duplex also reduces or inhibits the
porosity of the gold and gold alloy layers, and reduces oxygen
penetration of the metal layers. The reduction of the porosity of
the gold and gold alloys reduces or prevents undesired oxidation of
underlying metals. This in turn improves the solderability of the
gold and gold alloy surface finish and decreases contact
resistance.
[0049] The methods may be used to deposit the duplex of nickel and
nickel phosphorous on any suitable substrate. Typically such
substrates are metals or metal alloys. Suitable metals include, but
are not limited to, copper, iron, and their alloys, stainless
steel, and precious metals such as gold, platinum, palladium,
silver and their alloys. Typically, the substrates are copper,
copper alloys, iron and iron alloys. Suitable copper alloys
include, but are not limited to, copper-tin, copper-silver,
copper-gold, copper-silver-tin, copper-phosphorous-gold,
copper-zinc, copper-silver-magnesium, copper-iron-zinc,
copper-tin-nickel-silicon, copper-zirconium,
copper-iron-phosphorous-zinc and copper-nickel-silicon-magnesium.
Suitable iron alloys include, but are not limited to, iron-copper
and iron nickel. Such substrates include, but are not limited to
components of electrical devices such as printed wiring boards,
connectors, bumps on wafers, lead frames as well as passive
components such as resistors and capacitors for IC units.
[0050] The following embodiments of the invention are included to
further illustrate the invention and are not intended to limit its
scope.
EXAMPLE 1 (Comparative)
[0051] Three (3) copper-tin alloy lead frames were ultrasonically
cleaned at 65.degree. C. for 30 seconds in a cleaning solution
containing 100 g/L of RONACLEAN.TM. CP-100, which is a silicate
containing cleaning composition obtainable from Rohm and Haas
Company, Philadelphia, Pa., U.S.A.
[0052] After cleaning, each lead frame was then immersed into a
solution of 100 ml/L of technical grade sulfuric acid at room
temperature for 10 seconds. Each lead frame was then electroplated
with a nickel layer of 2 microns. The nickel bath used to plate
each lead frame had the formula disclosed in Table 1 below.
TABLE-US-00001 TABLE 1 COMPONENT CONCENTRATION Nickel Sulfamate 125
g/L Nickel Chloride 8 g/L Boric Acid 35 g/L Sulfonated Naphthalene
Compound 0.5 g/L Benzosulfimide Compound 5 g/L Aldehyde 0.5 g/L
Water Balance
[0053] The pH of the nickel bath was 4 and the temperature of the
bath was 50.degree. C. Each lead frame was then placed in a Hull
cell containing the electrolytic nickel bath. The lead frame
functioned as the cathode and the anode was a sulfur/nickel
electrode. The nickel bath was paddle agitated during deposition.
The apparatus was connected to a conventional rectifier. Current
density was at 10 ASD. Nickel deposition was done over a period of
90 seconds.
[0054] Two (2) of the nickel plated lead frames were then plated
with a nickel phosphorous layer 1 micron thick. The nickel
phosphorous bath used to plate the two lead frames had the
formulation as shown in Table 2 below. TABLE-US-00002 TABLE 2
COMPONENT CONCENTRATION Nickel Sulfamate 125 g/L Phosphorous Acid
50 g/L Boric Acid 35 g/L Digylceryl Ether 30 mg/L Salt of an Alkyl
Hydrogen Sulfate 1 g/L Water Balance
[0055] The nickel phosphorous bath had a pH of 1 and was at a
temperature of 60.degree. C. during nickel phosphorous deposition.
Nickel phosphorous was done in Hull cells with paddle agitation at
a current density of 10 ASD for one lead frame and 5 ASD for the
second lead frame. Electroplating was done on one lead frame for 60
seconds (10 ASD) and on the second lead frame for 30 seconds (5
ASD). The nickel phosphorous layer plated at 10 ASD had a
phosphorous content of 6 wt %, and the nickel phosphorous layer
plated at 5 ASD had a phosphorous content of 9 wt %. The
phosphorous content was measured by conventional energy dispersive
X-ray spectroscopy. A 1.times.1 mm area was measured.
[0056] Each of the three lead frames was then plated with a gold
layer 0.2 to 0.5 microns thick. The gold electrolyte included 12
g/L of gold ions from sodium gold sulfite and conventional gold
electrolyte additives. Gold plating was done at a current density
of 3 ASD for 4 seconds at 35.degree. C.
[0057] Each lead frame was then soldered with eutectic tin/lead
solder at 235.degree. C. The soldered lead frames were then steam
aged according to Military Specification 883C, Method 2003 which
involves steam aging at 95.degree. C. and 95% relative humidity for
8 hours. This simulated a shelf life of at least 6 months.
[0058] The zero crossing time was then determined for each lead
frame for solderability performance after steam aging. The zero
crossing time was determined with Menisco ST-50 solderability
tester. The zero crossing time for the lead frame without the
nickel phosphorous layer was 3.2 seconds, the zero crossing time
for the lead frame with the 6 wt % phosphorous content was 1.8
seconds, and the zero crossing time for the lead frame with the
phosphorous content of 9 wt % was 0.9 seconds. Accordingly, the
nickel and nickel phosphorous layers showed better solderability
performance than the lead frame without the nickel and nickel
phosphorous layers.
EXAMPLE 2 (Comparative)
[0059] Two (2) brass lead frames were ultrasonically cleaned at
60.degree. C. for 30 seconds in RONACLEAN.TM. CP-100 cleaning
solution. The lead frames were then immersed into a solution of 100
ml/L of technical grade sulfuric acid at room temperature for 10
seconds. Each lead frame was then electroplated with a 2 microns
layer of nickel from a nickel bath having the formula in Table 3
below. TABLE-US-00003 TABLE 3 COMPONENT CONCENTRATION Nickel
Sulfamate 125 g/L Nickel Chloride 8 g/L Aminoacetic Acid 30 g/L
Sulfonated Naphthalene Compound 1 g/L Benzosulfimide 5 g/L Aldehyde
0.5 g/L Water Balance
[0060] The pH of the nickel bath was 3 and the temperature of the
bath was 50.degree. C. Each lead frame was placed in a Hull cell
containing the electrolytic nickel bath. Nickel was deposited on
the lead frames at a current density of 10 ASD. Nickel deposition
was done over 80 seconds. The bath was paddle agitated during
electroplating.
[0061] One of the lead frames was then plated with a 1 micron layer
of nickel phosphorous using the bath disclosed in Table 4 below.
TABLE-US-00004 TABLE 4 COMPONENT CONCENTRATION Nickel Sulfamate 125
g/L Phosphorous Acid 50 g/L Boric Acid 30 g/L Aminoacetic Acid 25
g/L Sulfonated Naphthalene Compound 0.5 g/L Diglyceral Ether 35
mg/L Water Balance
[0062] The phosphorous content of the nickel phosphorous layer was
9 wt %. The pH of the bath was 1 and electroplating was done at
60.degree. C. The electroplating was done in a Hull cell with
paddle agitation. The current density was maintained at 5 ASD and
done for 20 seconds.
[0063] Each lead frame was then plated with a gold layer of 0.2 to
0.5 microns. The gold bath included 12 g/L of gold ions from sodium
gold sulfite, and conventional gold electrolyte additives. Gold
plating was done at a current density of 3 ASD for 4 seconds at
30.degree. C.
[0064] Each lead frame was then placed on a plastic holder in a
dessicator. Fuming nitric acid vapor with a concentration greater
than 65% was added to the dessicator for 1 hour at room
temperature. The nitric acid was then rinsed off using deionized
water. The gold surface of each lead frame was then placed under a
Nomarski microscope at 50.times. to examine the gold layers for any
pore formation. FIG. 1 shows the gold surface of the lead frame
which was not plated with the nickel phosphorous layer. Numerous
pores formed in the gold layer. In contrast, FIG. 2 shows the gold
layer on the lead frame which contained the nickel phosphorous
layer. Pore formation was sparse. Accordingly, the nickel and
nickel phosphorous layers under the gold layer was able to reduce
the formation of pores in gold.
EXAMPLE 3 (Comparative)
[0065] Three (3) brass lead frames were ultrasonically cleaned
using 100 g/L of ROANACLEAN.TM. CP-100 at 65.degree. C. for 30
seconds. After cleaning, each connector was then immersed into a
solution of 100 ml/L of technical grade sulfuric acid at room
temperature for 10 seconds. Each lead frame was then plated with a
nickel layer of 2 microns using the nickel electrolyte in Table 1
above. Nickel deposition was done in a Hull cell at a pH of 4 and
at a temperature of 50.degree. C. The current density was 10 ASD
and was done over 80 seconds.
[0066] Two (2) of the lead frames were then plated with a 1 micron
layer of nickel phosphorous. The nickel phosphorous electrolyte
used was the same as that disclosed in Table 2 above. The pH of the
electrolyte was 2 and nickel phosphorous deposition was done at a
temperature of 50.degree. C. The current density for one of the
plating baths was 10 ASD and the other was 5 ASD. Nickel
phosphorous deposition was done over 20 seconds for each bath.
[0067] All three of the lead frames were then electroplated with
gold in a Hull cell at a current density of 3 ASD for 4 seconds.
The gold electrolyte included 12 g/L of gold ions from gold sulfite
and conventional gold electrolyte additives. The temperature of the
bath was 30.degree. C.
[0068] Each of the gold plated lead frames was then steam aged
according to Military Specification 883C, Method 2003 which
involves steam ageing at 95.degree. C. and 95% relative humidity
for 8 hours. The lead frames were then tested for oxygen
penetration into the nickel layers, and nickel migration into the
gold layers.
[0069] Table 4 below discloses the results of the tests. The atomic
% of each element at different sputter depths was measured using a
conventional X-ray photoelectron spectrometer inside a ultrahigh
vacuum chamber. TABLE-US-00005 TABLE 4 Ni/Ni--P/Au Ni/Ni--P/Au
Ni/Au (7 wt % P) (10 wt % P) Depth (.ANG.) Au Ni O Au Ni O P Au Ni
O P 0 8.9 32.3 58.8 23.2 19.9 56.9 -- 8.1 36.3 55.6 -- 100 15.1
50.6 34.3 97.8 5.4 0.4 -- 77.1 17.7 5.2 -- 200 14.1 44.4 41.5 89.6
2.2 -- -- 88.7 7.3 2.7 1.3 300 6.2 54.1 39.7 89.6 9.3 -- 1.1 91.9
7.1 1.0 -- 330 5.4 54.4 40.42 58.1 37.5 -- 4.4 85.0 13.1 -- 1.9 360
4.9 55.4 39.7 27.6 66.0 -- 6.4 63.4 29.4 2.1 5.1 390 4.5 56.1 39.4
13.2 82.2 -- 4.6 42.6 48.0 1.0 8.4 420 4.5 56.2 39.3 7.4 88.1 --
4.5 26.5 63.4 1.9 8.2 450 4.2 56.7 39.1 5.2 91.1 -- 3.7 16.5 75.6
2.4 5.5 480 4.3 56.3 39.4 4.3 91.7 -- 4.0 11.0 82.4 1.9 4.7 510 4.2
56.9 38.9 3.7 92.8 -- 3.5 7.5 88.5 1.8 2.2 540 4.0 56.7 39.3 3.5
92.5 0.7 3.3 5.6 91.6 1.8 1.0 570 4.0 56.9 39.1 3.3 92.0 0.6 4.1
4.3 93.6 1.8 0.3 600 3.9 57.1 39.0 3.1 92.5 0.4 4.0 3.4 94.9 1.7 --
630 3.9 57.1 39.0 2.9 92.7 0.7 3.7 2.6 95.6 1.8 -- 660 3.9 57.5
38.6 2.9 92.7 0.4 4.0 2.3 96.0 1.7 -- 690 3.9 57.4 38.7 2.8 93.3
0.4 3.5 2.1 96.2 1.7 -- 720 3.9 57.4 38.7 2.8 93.5 0.3 3.4 2.1 96.2
1.7 -- 750 3.8 57.8 38.4 2.7 93.0 0.7 3.6 1.6 96.8 1.6 -- 780 3.9
57.4 38.7 2.6 93.1 0.4 3.9 1.5 96.7 1.8 -- 810 3.8 58.0 38.2 2.5
93.5 0.4 3.6 1.4 96.7 1.9 -- 840 3.7 58.0 38.3 2.6 93.3 0.4 3.7 1.3
97.0 1.7 -- 870 3.8 58.7 37.5 2.4 93.5 0.5 3.6 1.2 97.2 1.6 -- 900
3.6 58.1 38.3 2.5 93.0 0.3 4.2 1.2 96.9 1.9 --
[0070] The results in the table above show that the oxygen content
of the metal layers of the lead frames which were plated with a
nickel phosphorous layer were far below that of the lead frame with
just the nickel layer. The atomic% of oxygen in the metal layers of
the lead frame with the nickel phosphorous layer containing 10 wt %
of phosphorous had a high oxygen content of 5.2 atomic % at a depth
of 100 .ANG. and a low oxygen content of 0 atomic % at 330 .ANG..
The average atomic % oxygen for all depths was 4.1. The lead frame
with the phosphorous content of 7 wt % had a high oxygen content of
0.7 atomic % at 540 .ANG. and a low oxygen content of 0 atomic % at
200-510 .ANG.. The average oxygen content for all depths was 2.6
atomic %. In contrast, the lead frame without the nickel
phosphorous layer had a high oxygen content of 41.5 atomic % at 200
.ANG. and a low oxygen content of 34.3 atomic % at 100 .ANG.. The
average oxygen content for all depths was 36 atomic %. Thus, the
nickel phosphorous layers reduced oxygen penetration.
EXAMPLE 4
[0071] Example 2 is repeated except that an aqueous gold cobalt
alloy electroplating bath is used to plate a gold-cobalt layer on
the nickel or nickel phosphorous layer. The gold-cobalt alloy bath
includes 4 g/L of potassium gold cyanide, 1 g/L of cobalt sulfate
(0.25 g/L of cobalt ions) and 150 g/L of methylene phosphonic acid.
The pH of the aqueous gold-cobalt bath is 4. The pH of the bath is
maintained with potassium hydroxide. The bath is maintained at
40.degree. C. and the current density is 1 ASD. Electroplating is
done until a 0.2 to 0.5 microns layer of gold-cobalt is formed on
the nickel phosphorous layer of each lead frame.
[0072] Each lead frame is then placed on a plastic holder in a
dessicator and exposed to fuming nitric acid as in Example 2. The
gold-cobalt layer on the lead frame with the duplex is expected to
have fewer pores than the gold-cobalt layer on the lead frame with
just the nickel layer.
EXAMPLE 5
[0073] Example 4 is repeated except that the aqueous gold-cobalt
alloy electroplating bath includes 4 g/L of potassium gold cyanide,
1 g/L of cobalt as tetrapotassium salt of cobalt methylene
phosphonic acid, 124 g/L of tripotassium citrate, 29 g/L of citric
acid and 45 g/L of monopotassium phosphate. The pH of the bath is
maintained at 5, and the bath temperature is maintained to
35.degree. C. The current density is at 1 ASD and plating is done
to form a 0.2 to 0.5 microns layer of gold-cobalt on each lead
frame.
[0074] After treating each lead frame with fuming nitric acid, the
lead frame with the duplex layer is expected to have a gold-cobalt
layer with fewer pores than the gold-cobalt layer on the lead frame
without the duplex layer.
* * * * *